This invention relates to an amplifier circuit and, in particular, to a differential amplifier circuit having a high degree of common mode rejection.
Amplifiers embodying the invention are well suited for use in amplifier systems and, in particular, in amplifier systems suitable for amplifying very low amplitude signals riding on a much higher amplitude noise component.
There are many applications where signals to be detected and amplified are part of more complex signals. By way of example, one such application includes piezoelectric devices used as the sensors to sense the rate of flow of water in a pipe. The signals produced by the sensors may have a compound form and be of the type shown in the waveform of FIG. 1. A signal may include a large amplitude slowly varying component (identified as A1 in FIG. 1) on which is superimposed a low amplitude information signal (identified as B1 in FIG. 1) having a higher frequency than the slowly varying high amplitude commponent. The large amplitude component A1 is in effect an undesirable noise component which does not contribute to the flow rate measurement. The noise component A1 is generally offset with respect to the reference direct current (DC) level and may vary from less than a volt to 10 or more volts. The "true" or information signal component B1 which rides on the noise component A1 may have an amplitude in the microvolt range.
A problem with detecting and amplifying the desired information signal, B1, is that the high amplitude slowly varying noise component, A1, tends to mask the signal of interest. This problem may be overcome, in part, by applying the true signal and the noise component to one input of a differential amplifier and, if possible, the noise component to the other input of the differential amplifier. This does tend to reduce the effect of the noise components.
However, the high amplitude noise components produce large amplitude common mode voltages which when applied to the two inputs of the differential amplifiers designed to amplify the signals of interest tend to shift their operating range. Normally, a differential amplifier is designed to be operated at a certain optimum bias level and it is desirable to maintain the bias level for optimum operation. This is particularly so where very small signals have to be sensed and amplified. Therefore, a problem exists where a small signal rides on a large common mode signal. For example, in the system under discussion a differential signal derived from a pair of sensors and applied to a differential amplifier may have an amplitude of a few microvolts while riding on a common mode voltage which may vary over virtually the full operating voltage applied to the amplifier. As the common mode voltage applied to both inputs of a differential amplifier varies over the full range of the operating voltage the bias current in the amplifier may change drastically. This change affects the gain and the output drive capabilty of the amplifier and is highly undesirable.
Another problem is that the signals of interest tend to be of smaller amplitude than the offset of the amplifiers used to amplify the signals also masking or distorting the signals. Still another problem is that it is desired to have a system which can operate at very low voltage and power.
The problems discussed above are reduced in circuits embodying the invention by using differential amplifiers to process the signals and alternating current (AC) coupling the signals to differential amplifiers specially designed to have a high degree of common mode rejection.